Technically, the "official" was released on the Sammy Atomiswave arcade platform in 2006. Because the Atomiswave uses hardware similar to the Sega Dreamcast, it is generally not supported by older versions of MAME like 0.139u1, which focus on older hardware.
The ROM zip should contain (exact names/case matter for 0.139u1): metal slug 6 mame 0139u1 rom exclusive
Here is the controversial truth:
The term "exclusive" here likely refers to a that was unique around that MAME version: Technically, the "official" was released on the Sammy
use a frontend like LaunchBox with this old version — many expect newer MAME. metal slug 6 mame 0139u1 rom exclusive